Add unit conversion

[kdavies4]

Use the displayUnit by default but allow custom override in SimRes.get_values(), SimRes.get_IV(), etc.

Ideas for implementation:

• Replace the function f argument to SimRes.get_values(), etc. with a unit argument:
  • If default (None), then use displayUnit.
  • Accept string in Modelica notation (e.g., "m/s2"). The units may contain SI prefixes.
  • Accept values (e.g., m/s**2) where the values are available from a submodule (modelicares.units).
  • Accept functions (e.g., degC, dB), also from a submodule.
  • Build the units into the transformation from the data space to the axis space of matplotlib?
• Allow configuration of default units (via config file or code).
• Order of precedence: unit argument, default units, displayUnit attribute, unit attribute
• Also include constants in the units submodule (e.g., c/(m/s) should return 299792458).
• Each unit is a class that contains a dimension as well as a value. Use the dimension for dimension checking.
• List available units and constants in the documentation of modelicares.units.

[kdavies4]

Convert methods like min, max, mean, FV, value, and IV to properties. Remove the f argument to methods where no longer needed.

[kdavies4]

Unit conversion will be based on natu module (to be published soon).